Mel Slater's Presence Blog

Thoughts about research and radical new applications of virtual reality - a place to write freely without the constraints of academic publishing,and have some fun.

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I still find immersive virtual reality as thrilling now as when I first tried it 20 years ago.

07 November, 2014

What happens in your brain when your virtual body is threatened?

What happens in your brain when your virtual body is threatened?

Mar González-Franco, Tabitha C Peck, Antoni Rodríguez-Fornells, Mel Slater (2014)  A Threat to a Virtual Hand Elicits Motor Cortex Activation   Experimental Brain Research 232: 3. 875-887.

Figure 1. The experimental setup. Real: the participant wears a high resolution, wide field-of-view, stereo, head-tracked head-mounted display (NVIS SX111) and EEG cap (g.tec). Virtual top: the virtual reality showing the gender-matched virtual body spatially coincident with the participant’s actual body, and in the same posture. Virtual bottom: The two experimental conditions seen by the participant when looking towards the hand from a first person perspective: HAND - virtual hand stabbed by the knife; TABLE - virtual table stabbed by the knife (control condition).

When you wear a head-tracked wide field-of-view stereo head-mounted display and you look down towards your body you can see a life-sized virtual one instead. You look around and you see a reflection of that body in a mirror. In your whole life whenever you have looked towards your body you have seen it, likewise towards a mirror. Hence the simplest perceptual hypothesis for the brain to adopt is that this is your body.

In this study we looked at what happened when the virtual body was threatened. When someone anticipates that a knife might stab their hand that is resting on a table they would be likely to attempt to move the threatened hand out of the way. They would expect to feel considerable pain should the knife actually stab the hand. In this work we considered what happens when a person’s real body is visually substituted by a life-sized virtual body, and they see a threat or attack to a hand of this virtual body seen from first person perspective. Our experiment investigated brain activity in response to events that would cause pain to the observer were these events to occur in reality. Our contribution has been to  introduce a new technique for the study of pain observation, by using immersive virtual reality (IVR) for the scenario and stimulation, while recording brain activity with EEG.

Pain observation experiments typically present a series of pictures with hands or other extremities undergoing painful situations, and they compare the brain response of the participants to the activation produced by pictures where the same extremities do not undergo painful situations [1-4]. Many of these experiments present scissors and needles perforating the extremities as painful stimuli. A potential advantage of immersive virtual reality is that there is greater ecological validity, going beyond the presentation of two-dimensional, static stimuli. There is a life-sized, three dimensional virtual body seen in stereo, that visually substitutes the obscured real body of the participant and results show that this normally induces a whole body ownership illusion [5]. Our hypothesis was that harm to the virtual hand would be associated with positive changes in P450 in line with previous studies, and that this would be enhanced with illusory body ownership. We also investigated the mu band and readiness potential (RP).

While immersed in the virtual reality the 19 participants (10 female, right-handed) repeatedly experienced during 15 minutes two conditions in a within-group design: HAND where the knife stabbed the virtual right hand, and TABLE where the knife stabbed the table 15 cm away from the right hand (Figure 1). The experiment consisted of 70 trials repeating the HAND and TABLE conditions (30 HAND and 40 TABLE).

Both EEG and electromyography (EMG) were recorded using an gUSBamp  amplifier with a resolution of 30nV; the electrodes were set to cover the motor cortex area and surrounding: FC3, FC4, C3, C4, CP3, CP4 located according to the 10/20 standard EEG recording; the reference was set with an ear clip on the left ear lobe; the ground was positioned on the forehead; electrodes in the face measured ocular activity (EOG). Three EMG electrodes were placed in the flexor carpi ulinaris muscle of the right arm to measure whether participants moved their hand. All the electrodes were kept to impedances below 10 kΩ. The data was recorded with a sampling frequency of 512 Hz. After the exposure participants answered a questionnaire on a 1-5 Likert Scale where 1 was anchored to strong disagreement and 5 to strong agreement:

Ownership: I felt as if the hand I saw in the virtual world might be my hand.
Harm Hand: I had the feeling that I might be harmed when I saw the knife inside the hand.
Harm Table: I had the feeling that I might be harmed when I saw the knife outside the hand.
No Ownership: The hand I saw was the hand of another person.
Body Threat:  I saw the knife as a threat to my body.

Figure 2. EEG Recordings. Left: Grand averaged stimulus locked ERPs for six representative front, central and parietal electrode locations. A significant increase in the amplitude of the P450 is observed in the HAND condition mainly at C3 and CP3 locations. Baseline from [-200 ms to 0 ms], time 0 indicates the stimuli onset; a low pass filter 12Hz half-amplitude cutoff was applied. Right: (a) Time Frequency Evolution of the two conditions and the difference in the spectral activity. (b) Grand averaged 1-s short time power spectra calculated from EEG data (electrode C3) recorded. The baseline corresponds to the range [-1 to 0] seconds before the stimuli and the activity period corresponds to the range [0.7 to 1.7] seconds after the stimuli. Both the Baseline and TABLE frequency spectra show a peak in the mu-rhythm that is attenuated in the HAND condition. (c) Grand averaged Mu-rhythm (9-12Hz) Event Related Desynchronization for the C3 electrode. (d) Grand averaged Readiness Potential (C3-C4) subtraction between the brain activity in the two hemispheres shows movement preparation effects. Low pass filter 8Hz, half-amplitude cutoff.

Figure 3. Box plots showing the responses to the questionnaire. The thick lines are the medians, and the boxes are the interquartile ranges (IQR). Wilcoxon matched pairs sign-rank tests show differences between Ownership and No Ownership  (P < 0.0001); Harm Hand and Harm Table (P < 0.0002); Body Threat and Harm Table (P < 0.0003). Harm Hand and Body Threat ( P < 0.018).


• The results suggest that when a person is in an immersive virtual reality and has body ownership illusion towards a virtual body that apparently substitutes their own body, there are autonomic responses that correspond to what would be observed were the events to take place in reality. Overall automatic brain mechanisms –P450– were found in this variation of the classical pain observation experiment, which is consistent with previously reported results.

• The results cannot be explained as participants experiencing empathy towards another person since they witnessed attacks to their co-located virtual body and both subjective and objective data suggest that they experienced this as an attack on their own body.

• The results support our initial hypothesis that a threat to a virtual hand, towards which the participant has an illusion of ownership, would significantly produce a harm prevention effect (the Readiness Potential (C3-C4) and oscillatory movement-related components, the mu-ERD), such as trying to move it away from the source of the harm. The questionnaire also confirmed high levels of ownership over the virtual body.

• The correlation between the automatic brain mechanisms –P450– and the subjective illusion of ownership suggests  a potentially new measure of virtual embodiment.

1. Avenanti, A., et al.,. NeuroImage, 2006. 32(1);  
2. Bufalari, I., et al., Cerebral Cortex, 2007. 17(11); 
3. Fan, Y. and S. Han, Neuropsychologia, 2008. 46(1);  
4. Li, W. and S. Han, Neuroscience Letters, 2010. 469(3);
5. Slater, M., et al., PLoS ONE, 2010. 5(5).

Funded by European Union FP7 IntegratedProject VERE (#257695);  FI-DGR predoctorate grant from the Catalan Government co-funded by the European Social Found (EC-ESF); Spain MICIN (PSI2011-29219);  ERC project TRAVERSE (#227985).